Got burned myself the other day - ended up paying someone's rent in the UK (twice) and their gas bill. Not very trusting of these .pdf download sites as a result. Big warning sign is if you have to pay. Don't!
My only saving grace was I had a sacrificial credit card for use on the internet (which I then had to sacrifice). It is not linked to anything else and I only ever use it for the internet. Gone now, and I've got another.